The reproductive system of a female, unlike men, shows regular cyclic changes that teleologically may be regarded as periodic preparation for pregnancy and fertilization. In primates and humans, the cycle is a menstrual cycle, and its most conspicuous feature is the periodic vaginal bleeding that occurs with the shedding of uterine mucose (menstruation). The length of the cycle is notoriously variable, but an average figure is 28 days from the start of one menstrual period to the start of the next. By common usage, the days of the cycle are identified by number starting with the first day of menstruation. It begins at puberty, ranging from the ages of 10 to 16, and ends at menopause at an average age of 51.
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